Airplane flight relies on various control surfaces, sensors, transducers, and probes to function properly. Many of these critical components are externally mounted on aircraft, and are therefore subject to several environmental challenges. Accumulation of ice, snow, or frost on leading edges and horizontal and vertical stabilizer surfaces may alter their aerodynamic shapes, which may ultimately lead to loss of control or insufficient lift to keep the aircraft airborne. Accumulation of ice, snow, or frost on sensors, transducers, and probes may lead to erroneous data readings, which may also have a deleterious effect on the functioning of the aircraft, which may be catastrophic.
To prevent ice formation in flight, hot bleed air from one or more engine compressors is used to melt, and therefore prevent dangerous build up, of ice, snow, or frost on the critical components exposed to the external environment. However, air ducts are required to convey the hot bleed air from the engine compressors to the critical components, thereby increasing the weight, engine thrust requirements, and fuel burn. For this reason, hot bleed air heating systems have been replaced by electric deicing heaters on newer aircraft. These deicing heaters are composed of resistive coils or elements embedded in metal cases, metal parting strips, or carbon fiber composite structures, which are typically incorporated into the sensors, transducers, and probes to form structurally integrated units.
Salt, sand, pollutants, volcanic ash, ice, moisture, insects, plus shock, vibration, pressure, temperature cycles, etc., degrade the performance of these deicing heaters, thereby reducing the component's service life to four to five years. From field returns, it has been discovered that, over time, corroded deicing heaters, due to degraded insulation that loses its electrically insulative properties, develop conductive paths to the heater case, which is bonded to the chassis of the aircraft. Such conductive paths shunt away the heating current, allowing ice, snow, or frost to form, rebuild, and accumulate in flight.
Airplane onboard systems monitor the electrical current of these deicing heaters to determine the existence of a fault condition. A warning indication is generated based on monitoring the operating current of these heaters. However, due to manufacturing tolerance, supply voltage variations, and transient behaviors of the deicing heater current, existing onboard monitoring systems are designed to operate with large fluctuations of the heater current under all operating conditions. For example, as the temperature of the heater increases, the heater resistance increases, thereby decreasing the heater current. As such, current onboard monitoring systems are designed to only determine two distinct fail conditions based on two predetermined current thresholds: a “fail-open” if measured heater current reaches its minimum threshold (e.g., 30 mA) and a “fail-short” if measured heater current reaches its maximum threshold (e.g., 3 A). In fact, cases for “fail-open” can be either “cable-open,” or “connector-open,” or “heating element-open,” and cases for “fail-short” can be either “cable-short” or “connector-short.” However, many failed or failing electrical deicing heaters draw electrical current in the range between the minimum and maximum failure thresholds, and are therefore interpreted as being in a “no-fault” condition by the on-board monitoring system.
Thus, today's on-board monitoring systems are unaware of the timely deteriorations of electrical deicing heaters until the current magnitude gradually becomes sufficiently high to trip an upstream circuit breaker, which results in the identification of a “fail-short” condition, or the deicing heater is burned open, resulting in the identification of a “fail-open” condition. Thus, not until the deicing heater has completely failed, which typically occurs in-flight, will there be any indication that a failure has occurred. As such, the existing on-board monitoring systems cannot recognize a large percentage of deicing heaters that have failed in service. A degraded electrical deicing heater will no longer perform its intended function, causing instrument errors, disagreement in altitudes and airspeed from these “no-fault” probes, which may lead to cockpit confusion and aircraft stall absent intervention.
For example, wide-body aircraft typically have multiple probes measuring airplane's air speed, total air temperature, and angle-of-attack. These external mounted sensors often have embedded electrical deicing heaters that respectively receive power from the left, center, and right control panels in the cockpit, and provide data to the corresponding onboard flight control systems. With respect to air speed probes, failure of one or more of the electrical deicing heaters may cause substantially different air speed readings among these probes. There is no flight deck warning-indication until an air-data split, i.e., airspeed readings for captain and first officer are in disagreement due to an icing event. Upon noticing an air-data split, airline pilots are trained to first disengage the autopilot and auto-throttle, fly the aircraft manually, and land the airplane at a nearby airport. When ice forms and melts during flight, intermittent errors often result in the pilots' decision for Air Turn Back (ATB). Since the 1990s, the worldwide wide-body fleet has had numerous incidents of such event occurring in-flight. On the ground, maintenance crew will remove these air probes and measure the insulation resistance of each to determine which one is faulty. To eliminate failure of air data probes in-flight, airlines are forced to shorten the removal and replacement schedule, which may be as short as 13,000 flight hours or three years for the wide-body fleet. As such, air data probes are typically prematurely replaced even when they are properly functioning. This costly issue has been frequently raised by airlines, since the early 1990s.
There, thus, remains a need for an on-board monitoring system that monitors and reports the trend in the health status of critical components, including electrical deicing heaters, detects the failure of critical components in advance, and annunciates cockpit warnings at an early stage before complete failure of the critical components (e.g., before airspeed disagreement).